Split 50/50 between the guy who had the idea and the guy who actually makes it happen?
Ideas are not worth much. I'd buy someone a beer in return for an idea, if it was a particularly good one. Much more valuable is the interaction design (which involves hundreds of hours of sketching interfaces, trying mockups on the phone, drawing out common workflows and routes through your app), and the coding itself (a similar number of hours breaking the problem down into smaller steps, and wrestling with annoying bugs).
If by idea you mean something that can be summed up in two sentences, like "An app to manage your academic references, that lets you put in an author and date and it'll search google scholar for your article", then it's not worth anything much. If you mean something more elaborate, like the interaction design process I described above, then it might be valuable, if you're any good at it. But it's not a quick process: I'd expect many weeks' worth of work.
Sorry for a bit of a rant. I've just spent the entire day wrestling with a bug relating to reordering the rows in a table view, so I'm a bit annoyed at the perception that an idea is as valuable as the work of a good programmer.
Amorya